Support for callables in env no longer works about kubespawner

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[moschlar]

This currently bites me too on trying to upgrade from z2jh 2 to z2jh 3...

Maybe a possible workaround would be to use a Spawner.pre_spawn_hook!?

[moschlar]

Although from reading the code, I can not figure out why it does not work in the first place (i.e. why is https://github.com/jupyterhub/jupyterhub/blob/ca6032381aa137f03cb9349265fa74ed5e4c5c19/jupyterhub/spawner.py#L1101 apparently not relevant?)...

[manics]

KubeSpawner calls the parent function to evaluate callables as you've pointed out:

kubespawner/kubespawner/spawner.py

Line 2161 in f99b98f

env = super().get_env()

but in addition it adds template expansion of values, which is causing the problem:

kubespawner/kubespawner/spawner.py

Line 2169 in f99b98f

env.update(self._expand_all(self.environment))

I think the correct fix is to only expand non-callable values (since if you're using a callable you most likely don't want the result to be interpreted as a template).

[moschlar]

@manics I still don't get why the template expansion should cause this - it should ignore anything that is not list, dict or str, shouldn't it?

[consideRatio]

I really appreciate this issue writeup @PerilousApricot - thank you! My favorite part is the inclusion of the docs reference that makes it definite its a bug in the project!

And, according to the docs, this should be supported:

The environment configurable should be set by JupyterHub administrators to add installation specific environment variables. It is a dict where the key is the name of the environment variable, and the value can be a string or a callable. If it is a callable, it will be called with one parameter (the spawner instance), and should return a string fairly quickly (no blocking operations please!).

Thank you @manics for the fix and thank you @jabbera and @moschlar for reporting on this as well!